Spontaneous Electrical Activity of Sea Urchin Lantern Protractor Muscle

AbstractThe many different muscles of sea urchins are used to control tube foot and spine movement, feeding, excretion, respiration, and gamete release. Unlike in most other animals, the delineation between skeletal and smooth muscles in sea urchins is not clear cut, with many muscles showing characteristics of both muscle types. To further our understanding of sea urchin muscle function, we sought to characterize the electrical properties of protractor muscles of the Aristotle's lantern. Aristotle's lantern comprises a complex of multiple different muscles, ligaments, and calcite ossicles and is primarily involved in feeding. Within the lantern, antagonistic muscle pairs of protractor and retractor muscles act to raise or withdraw the lantern, respectively. The protractor muscles are unstriated, similar to smooth muscle, but connected to the calcite skeleton, as with skeletal muscles. We isolated single muscle cells from the protractor muscles and measured their membrane potentials and found that they generate spontaneous currents at a frequency that ranged from 25 to 30 Hz, a functional property found in many smooth muscles. Furthermore, these currents occurred in the absence of any extraneous cellular or chemical input. Measurement of the reversal potential of the currents under control and ion-substituted conditions suggests that they may be due to the activity a nonspecific cation channel or the synchronous activity of K⁺ and Na⁺ channels.